1. Field of the Disclosure
The present disclosure relates to high-power optical transmission using an optical fiber.
2. Discussion of the Background Art
Along with progress of high-output lasers, application to industrial processing such as welding using laser light is widely used. Especially, in recent years, high-output fiber lasers with an output of as high as 10 kW are developed and are expected to be used in medical and industrial fields. With such high-output fiber lasers, for example, as disclosed in Non Patent Literature 1, a core area is expanded in a short optical fiber of several meters or shorter, so that output power limitation due to non-linearity is relaxed. In laser processing, a beam quality of emitted light significantly affects processing efficiency. Since the beam quality strongly depends on a mode state of the emitted light, an optical fiber enabling single mode transmission is used in the fiber laser.
Furthermore, as disclosed in Non Patent Literature 2, an optical fiber is connected to an emission end of the above-described high-output laser to be applied to remote welding processing. In this case, the beam quality at the emission end is affected by an excitation state of a high-order mode in the connected optical fiber. Therefore, if a multimode optical fiber with a large core area is connected as a transmission optical fiber, high-power light of several kW may be transmitted over several tens of meters or longer, but the beam quality at the emission end is low. In order to improve the beam quality, it is necessary to reduce the number of propagation modes, but generally, in a structural design of the optical fiber, there is a trade-off relationship between the reduction in the number of propagation modes and the expansion of the core area, so that the power which may be transmitted is limited.